Re: [PATCH 2/9] KVM: x86: simplify read_emulated

Xiao Guangrong
2012-07-20T13:16:08Z

On 07/20/2012 06:58 PM, Marcelo Tosatti wrote:
> On Fri, Jul 20, 2012 at 10:17:36AM +0800, Xiao Guangrong wrote:
>> On 07/20/2012 07:58 AM, Marcelo Tosatti wrote:
>>
>>>> - }
>>>> + rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
>>>> + &ctxt->exception);
>>>> + if (rc != X86EMUL_CONTINUE)
>>>> + return rc;
>>>> +
>>>> + mc->end += size;
>>>> +
>>>> +read_cached:
>>>> + memcpy(dest, mc->data + mc->pos, size);
>>>
>>> What prevents read_emulated(size > 8) call, with
>>> mc->pos == (mc->end - 8) now?
>>
>> Marcelo,
>>
>> The splitting has been done in emulator_read_write_onepage:
>>
>> while (bytes) {
>> unsigned now = min(bytes, 8U);
>>
>> frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
>> frag->gpa = gpa;
>> frag->data = val;
>> frag->len = now;
>> frag->write_readonly_mem = (ret == -EPERM);
>>
>> gpa += now;
>> val += now;
>> bytes -= now;
>> }
>>
>> So i think it is safe to remove the splitting in read_emulated.
>
> Yes, it is fine to remove it.
>
> But splitting in emulate.c prevented the case of _cache read_ with size
>> 8 beyond end of mc->data. Must handle that case in read_emulated.
>
> "What prevents read_emulated(size > 8) call, with mc->pos == (mc->end - 8) now?"

You mean the mmio region is partly cached?

I think it can not happen. Now, we pass the whole size to emulator_read_write_onepage(),
after it is finished, it saves the whole data into mc->data[], so, the cache-read
can always get the whole data from mc->data[].